This invention relates generally to television signal transmission systems and specifically to so-called MMDS (microwave multipoint distribution systems).
MMDS distribution systems carry over-the-air television type signals in a restricted frequency range. The frequency range of the 6 MHz MMDS channels is 2150 MHz to 2686 MHz. A response signal arrangement is provided for in the channel frequency allocations whereby the MMDS receiving site may communicate back to the transmitting site. These systems are well known in the art and generally embody a pole-mounted block converter for downconverting the received analog MMDS channels to corresponding signal frequencies that the MMDS decoder is capable of processing. The downconverter includes a fixed frequency local oscillator operating at 2278 MHz that is generally in a crystal controlled phase locked loop (PLL) circuit arrangement for supplying a local oscillator output signal to a mixer that receives the input MMDS channels. The response signal is generated by a separate FM transmitter or may be carried over a separate telephone line. Ideally, the MMDS block converter oscillator is immune to environmental conditions to enable a highly stable 2278 MHz local oscillator output. In practice, the local oscillator output cannot be stabilized under the adverse environmental conditions in which it must operate. Even with a crystal controlled phase locked loop, and with the relatively reduced stability requirements of an analog signal system, difficulty is experienced with stability. Under these conditions, the oscillator is not sufficiently stable to permit generation of the response frequency, which is different from the MMDS signal frequency and must be precisely controlled. In the present analog signal environment, the art has failed to produce a simple, stable, low cost, two-way MMDS system that would permit the response signal to be radiated back to the transmission site via the receiver antenna. Further, in the rapidly forthcoming digital signal transmission systems with their much more stringent stability requirements, a need exists for a cost effective two-way MMDS communications system.